Packages: Adding RTL CSS support to the packages CSS

[youknowriad]

This PR adds RTL support to the packages CSS which allows us to drop the root package folders for all the packages that expose CSS files.

There's a new Webpack step copying the built CSS files from the packages folder into the build folder used by the WordPress registered styles.

Testing instructions

• Check that the nux RTL CSS file is generated properly in build-style and copied to the global build folder.

[gziolo]

Should we merge #8093 and refactor to use the asynchronous code?

[youknowriad]

I don't really care which one goes first but yeah, we should align

[gziolo]

Nice and clean 👍

[gziolo]

Almost there, in case of npm run build - output files aren't minified.

[gziolo]

It works great, many thanks for fixing it.

🎉

[aduth]

Do we still need the WebpackRTLPlugin in the default configuration after this change? I'm trying to understand how the plugin is being used, because it appears to be dead code (per documentation, requires some complementing text extract plugin).

gutenberg/webpack.config.js

Lines 50 to 54 in a9f6a1e

	// Create RTL files with a -rtl suffix
	new WebpackRTLPlugin( {
	suffix: '-rtl',
	minify: defaultConfig.mode === 'production' ? { safe: true } : false,
	} ),

It's also the sole plugin we use which hasn't been updated for Webpack 4 and would break with the upcoming Webpack 5 release.

(node:54786) DeprecationWarning: Tapable.plugin is deprecated. Use new API on .hooks instead

[youknowriad]

I think when we merged this PR, we were in a mixed where not all the packages were in the packages folder but I think now it not necessary anymore.

[aduth]

Okay, I'll plan to push up a pull request in the next day or so.

[aduth]

Just to make sure it's tracked, I created an issue at #15146. I expect to find some time to look at this on Friday, and have assigned myself accordingly.